Organic light-emitting diodes (OLEDs) have great potential in implementation of novel optoelectronic devices, such as flat panel displays and lighting applications, because of the synthetic diversity of organic semiconductors, low manufacturing costs, and high-performance optical and electrical properties.
In order to improve the emitting efficiency of organic light-emitting diodes, a variety of material systems based on fluorescent and phosphorescent materials have been developed. The use of fluorescent materials in organic light-emitting diodes has been highly reliable, but its internal electroluminescence quantum efficiency is limited to 25% under the excitation of electric field, because an exciton has a branch ratio of the singlet excited state and triple excited state of 1:3. In contrast, organic light-emitting diodes using phosphorescent materials have achieved almost an internal luminescence quantum efficiency of 100%. However, phosphorescent OLED has a significant problem, the Roll-off effect, that is, luminous efficiency decreases rapidly as the current or brightness increases, which is particularly unfavorable for high-brightness applications.
So far, the phosphorescent material with practical use value is iridium and platinum complexes. Such raw materials are rare and expensive, while the synthesis of complex is complicated, resulting in high costs. In order to overcome problem of rare and expensive raw materials and complicated synthesis of iridium and platinum complexes, Adachi proposed the concept of reverse intersystem crossing so that organic compounds can be used instead of metal complexes to achieve high efficiency comparable with phosphorescent OLEDs. However, most organic compounds with TADF adopt the form of donor groups connected with electron-deficient or electron-acceptor groups, resulting in the complete separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbit (LUMO), which reduces the difference (ΔST) between the organic compound singlet (S1) and triplet (T1), but also leads to a decrease of the resonance factor (f) of the organic compound and further causes the decrease of the fluorescence quantum efficiency of the organic compound.
In addition, the life of such OLED devices are yet to be improved.
Therefore, the existing technology, especially the solution for the material, needs to be improved and developed.